1. Field of the Invention
The present invention relates generally to a printing hammer driving mechanism in a printing apparatus. More particularly, a hammer driving mechanism which prevents double striking at the time of erasing a character is disclosed.
2. Description of the Related Art
Printing devices equipped with hammer mechanisms that strike type elements of a type wheel hard are generally known. A printing apparatus of this type is disclosed in Published Unexamined Japanese Utility Model Application No. 63-108741. As seen in FIG. 12, such an apparatus has a DC hammer driving motor 51 that is driven on the basis of the operation of character keys or a correction key on a keyboard. Solenoid 52 is then excited (energized), so as to move an armature 53 to an unlock position from a lock position. A hook 60 provided at an upper end of the armature 53 engages a projection 61 of a rotary body 54 when the armature 53 is in the locked position, thus locking the rotary body 54. The hook 60 disengages the projection 61, when the armature 53 is moved to the unlocked position, thus unlocking the rotary body 54. This releasing of the lock rotates the rotary body 54, with a cam 55 rotating together with the rotary body 54. The rotation of the cam 55 causes a crank 56 to drive a printing hammer 57 to print a character through a print ribbon 58 or erase a character through a correction ribbon 59.
The solenoid 52 is deenergized during one revolution of the rotary body 54, moving the armature 53 again to the locked position. At a hold position, the hook 60 catches the projection 61, which has reached there with a slight delay after movement of the armature 53 to the locked position, stopping the rotation of the rotary body 54. This stops and holds the printing hammer 57 via the cam 55 and crank 56, which prepares the apparatus for the next key operation.
The hammer driving motor 51 also drives a winding spool during the printing operation to wind the print ribbon around the spool by a length corresponding to one character. The load applied to the drive motor 51 at the printing time is therefore greater than the load at the erasing time. Accordingly, the rotational cycle of the drive motor 51 or the rotary body 54 is faster at the erasing time than at the printing time.
If the time that the solenoid is energized is constant during the erasing action, the armature 53 may return to the locked position with a slight delay. This can allow the rotary body 54 to rotate to cause the projection 61 to reach the hold position. Thus, it would be too late for the armature 53 to catch the rotary body 54. In such a case, the hook 60 cannot catch the projection 61, which permits the rotary body 54 to keep rotating. Consequently, the cam 55 drives the printing hammer 57 via the crank 56 to perform a second hammering operation.
To overcome this shortcoming, the energizing time for the solenoid 52 is set such that the timing for the armature 53 to return to the locked position is slightly faster than the timing for the rotation of the rotary body 54 to position its projection 61 at the hold position, thereby allowing the hook 60 to catch the projection 61 in time to surely lock the rotary body 54. However, even with the above structure, these two timings may change due to a variation in rotational torque of the drive motor 51 with passage of time or an increase in the magnetic remanence of the solenoid 52. When the return timing of the armature 53 becomes equal to, or delayed with respect to the timing of the rotary body's return to the hold position, an extra hammering operation will be undesirably performed during the erasing cycle, as in the previously described case.